Inject config environment access (#293)

[lodyai]

Summary

This branch completes the issue #293 environment-access refactor for early CLI
configuration resolution. It introduces an EnvProvider port, threads it
through explicit config selection and early diagnostic JSON resolution, and
removes the remaining direct process-environment dependency from
collect_diag_file_layers.

Closes #293.

Review walkthrough

• Start with src/cli/config_merge.rs to review the EnvProvider port, the StdEnvProvider adapter, and the provider-aware diagnostic resolver.
• Review src/cli/config_merge_tests.rs and tests/cli_tests/merge.rs for map-backed environment doubles and malformed NETSUKE_DIAG_JSON coverage.
• Finish with docs/developers-guide.md and docs/netsuke-design.md for the documented port boundary and OrthoConfig discovery caveat.

Validation

• cargo test --workspace config_merge: passed.
• make check-fmt: passed.
• make lint: passed.
• make test: passed.
• make markdownlint: passed.
• make nixie: passed.
• coderabbit review --agent: completed with zero findings on the main implementation pass.

Notes

make fmt was run after the documentation edit. Its Rust formatting step
completed, but the target exits non-zero in the Markdown formatting phase
because the underlying markdownlint --fix command reports repository-wide
line-length findings in pre-existing untouched documents. The formatter also
rewrote unrelated Markdown files, so those unrelated edits were restored. The
actual validation gates make check-fmt, make markdownlint, and make nixie
all pass on this branch.

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✨ Finishing Touches

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• Create PR with unit tests
• Commit unit tests in branch issue-293-refactor-collect-diag-file-layers

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